don't know if this helps the discussion any, went for a short
tour this week. qingming (tombsweeping) festival, had four
days off, did an ocean-mountains-ocean circuit. 400km on
the new 17.7 kg (39-pound!) touring rig. 23" steel frame,
full fenders, B17 clone saddle, 700*45 tires. carried 7kg,
just under 15 pounds, in the rear bags.

9-spd transmission has a 22-32-40 crank and 11-36 cassette,
which gives a range of 17-91 gear inches.

this here is a small island, flat on the edges, tall and pointy
in the middle. first two days mostly uphill, third day all
downhill, final day a cruise along the beach.

not sure about the gradient without researching, but i'd
put it at 8-10% for long stretches. nothing like the 40-km
long 12% grades in yunnan, but whatever.

anyhooooo, toodling uphill, i found the lowest cadence i
could comfortably maintain without getting too "twitchy"
was about 60 rpm. with 700*45 tires and 22:36 gearing,
that was 4.91 km/h (3.05 mph) without getting winded
or having an excessively increased heart rate. basically,
a "do it all day" pace. another 10-15 pounds in the bags
would not have made much difference.

it was possible with a bit more concentration to manage
55 rpm, at 4.50 km/h (2.80 mph), just not as relaxing.